| Soil microorganism is an important part in forest ecosystem, which a play key role in managing soil nutrient budget, adjusting soil physical and chemical balance, clearing soil toxic substances, and as well as assisting plants in nutrient absorption. Nitrogen (N) is considered to be a limiting nutrient factor for net primary productivity in temperate forest ecosystem, which affects the changes of material and energy of soil ecosystem. Therefore, the study of the influence of N addition on the content and community structure of soil microorganism has a great meaning in understanding the response of soil ecosystem to environment changes. In this study we focused on the responses of soil microbial biomass and structure in plantations of Pinus koraiensis, Picea koraiensis, Larix gmelinii, Phellodendron amurensis, Fraxinus mandschurica, Juglans mandschurica under N fertilization treatment by the method of Phospholipid Fatty Acids (PLFA), and discussed the factors which affect soil microbial community structure combining with the analysis of soil physical and chemical properties. The results were as followed:(1) In the sample plots, N fertilization enhanced the contents of NH4+-N and NO3--N in six plantations, particularly increasing in NC3--N. N fertilization reduced soil pH, indicating a strong soil nitrification in six plantations. In addition, N fertilization didn’t significant influence on the contents of soil total carbon (TC) and total nitrogen (TN), which means that short-time N addition couldn’t change their allocation patterns. By contrast, the content of total phosphorus (TP) in six plantations significant decreased under fertilization treatment, probably due to greater uptake for P as increasing soil N availability.(2) There were significant seasonal variations in soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) in six plantations, with the variation patterns of spring< summer< autumn. Although N addition significant decrease soil MBC and (MBN) in each growing season, but it didn’t change their seasonal patterns, showing that N addition had an influence on soil microbial biomass in our study plantations.(3) PLFA analysis showed that N addition had greatly influences on soil microorganisms, resulting in the significant decrease in the total quantities of soil microorganisms as well as bacteria in six plantations, but the influences was greater in coniferous stand than in broad-leaf stand. N addition had significant influence on the quantity of funguses in coniferous stands rather than in broad-leaf forest stands. In addition, the quantity of soil actinomycetes in all the six plantations had not changes between N addition and control treatments.(4) N addition significantly changed soil microbial community structure in the six plantations, resulting in the significant decrease the ratio of soil funguses-bacteria, which is mainly due to the decrease of fungus quantity. In addition, N addition also changed bacterial community structure, resulting in the decrease of the ratio of gram-positive bacteria to gram-negative bacteria, suggesting that N addition relieves the limitation of environmental conditions to gram-negative bacteria.(5) The correlation analysis among the microbial biomass with soil physical and chemical factors in six plantations showed that pH is the most important factor affecting the microbial biomass, suggesting that the levels of available N was not a factors which influence the activity and content of soil microorganism directly. However, the reduction soil pH values and enhancement soil acidification caused by N addition in six plantations con not be overlooked. |
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